Glutathione S-transferases (GSTs) are a family of enzymes that play an important role in detoxification by catalyzing the conjugation of many hydrophobic and electrophilic compounds with reduced glutathione. Based on their biochemical, immunologic, and structural properties, the soluble GSTs are categorized into 4 main classes: alpha, mu, pi, and theta. The glutathione S-transferase pi gene (GSTP1) is a polymorphic gene encoding active, functionally different GSTP1 variant proteins that are thought to function in xenobiotic metabolism (i.e., the metabolism of environmental mutagens and carcinogens) and may play a role in susceptibility to cancer. More recent experiments have suggested that differential expression of GSTP1 also contributes to the sensitivity of xenobiotics in the substantia nigra and may influence the pathogenesis of reactive oxygen species-induced neurological disorders such as Parkinson’s disease. CpG island hypermethylation of the GSTP1 promoter leading to the silencing of the GSTP1 gene has also been linked to cancer.